Two-wire signal transmission system

Abstract

A signal transmission system of the type used in process control systems for conveying a signal between a field telemetering instrument and a central controller, or between a central controller and a process control device. The signal transmission system is arranged with a transmitter accepting an input signal, a transmission line for carrying a transmission signal, and a receiver receiving the transmission signal and developing an output signal. In accordance with the invention, converter means in the transmitter develop a pulse width signal with a duration corresponding in a predetermined manner to the value of the input signal, and means in the transmitter provide the transmission signal (e.g., a current signal) with two states (e.g., opposite polarities), one of which has a duration corresponding to the duration of the pulse width signal. Means in the receiver provide the output signal with a duration corresponding to the duration of the one transmission signal state, which corresponds in turn to the value of the input signal, and can be used for conversion to digital or analog form. Means in one of the transmitter or receiver supplies power for the transmission signal, and means in the other of the transmitter or receiver derives operational power, e.g., for the telemetering instrument, from the transmission signal. The signal transmission system thus is able to convey signals with less susceptibility to noise, while at the same time retaining the ability to convey operational power.

We claim: 
     
       1. A signal transmission system of the type used in process control systems for conveying a signal between a field instrument and a central controller, or between a central controller and a process control device, comprising: a transmitter arranged to accept an input signal;   converter means in the transmitter for developing a pulse width signal with a duration corresponding in a predetermined manner to the value of the input signal;   means in the transmitter for providing a transmission signal with two states, said means being responsive to the converter means to cause the duration of one of the transmission signal states to correspond to the duration of the pulse width signal;   a transmission line for carrying the transmission signal;   a receiver for receiving the transmission signal;   means in the receiver for developing an output signal with a duration corresponding to the duration of the one transmission signal state, whereby the duration of the output signal corresponds in said predetermined manner to the value of the input signal;   means, in one of the transmitter or receiver, for supplying power for the transmission signal; and   means, in the other of the transmitter or receiver, for deriving operational power from the transmission signal;   whereby the signal transmission system is able to convey signals less affected by noise, and is also able to convey operational power.   
     
     
       2. A signal transmission system as claimed in claim 1 wherein the means for providing a transmission signal with two states comprises means for generating a current signal with two polarities, means in the transmitter and receiver connected to the transmission line for providing two different current loops for the two different polarities of the current signal, and switch means in one of said loops responsive to the converter means to cause the duration of the current flowing in that loop to correspond to the duration of the pulse width signal. 
     
     
       3. A signal transmission system as claimed in claim 2 wherein said loops include diode circuits arranged to control the polarity of current in the loops, and to detect and indicate the flow of current in the loops. 
     
     
       4. A signal transmission system as claimed in claim 3 wherein said diode circuits comprise a diode photo-coupled to a transistor. 
     
     
       5. A signal transmission system as claimed in claim 1 wherein the input signal is an analog signal and wherein the converter means develops a pulse width signal having a duration proportional to the value of the analog input signal. 
     
     
       6. A signal transmission system as claimed in claim 1 wherein the transmission signal is a current signal with different polarities forming its two states, and wherein the means in the receiver for developing an output signal with a duration corresponding to the duration of the one transmission signal state comprises means for detecting and indicating the flow of current in the polarity corresponding to the one state. 
     
     
       7. A signal transmission system as claimed in claim 6 wherein the detecting and indicating means comprises a diode circuit for controlling the polarity of current flow and for detecting and indicating the flow of current therethrough. 
     
     
       8. A signal transmission system as claimed in claim 7 wherein the diode circuit comprises a diode photo-coupled to a transistor. 
     
     
       9. A signal transmission system as claimed in claim 1 wherein the means for supplying power for the transmission signal is located in the receiver, and the means for deriving operational power from the transmission signal is located in the transmitter. 
     
     
       10. A signal transmission system as claimed in claim 1 wherein the means for supplying power for the transmission signal is located in the transmitter, and the means for deriving operational power from the transmission signal is located in the receiver. 
     
     
       11. A signal transmission system as claimed in claim 1 wherein the transmission signal is a current signal having different polarities as its two states, and wherein the means for deriving operational power from the transmission signal further comprises rectifier means for supplying a unidirectional current from the bipolar transmission signal. 
     
     
       12. A signal transmission system as claimed in claim 1 further comprising time base circuit means for generating a steady pulse signal to time a cycle of operation of the signal transmission system, and the signal transmission system is arranged to convey the one transmission signal state with a duration corresponding to the value of the input signal during each cycle of operation as determined by the time base circuit means. 
     
     
       13. A signal transmission system as claimed in claim 1 wherein the transmission signal is a current signal having opposite polarities as its two states, the transmitter and receiver comprise means forming a first current loop of one polarity through the transmission line and means forming a second current loop of opposite polarity through the transmission line, the means for forming the first current loop of said one polarity containing switch means responsive to the converter means for causing the duration of the one polarity of transmission signal current to correspond to the duration of the pulse width signal, the signal transmission system further comprising time base circuit means for determining a cycle of operation and means for switching the transmission signal current into said second current loop of opposite polarity for the remainder of a cycle of operation as determined by the time base circuit means. 
     
     
       14. A signal transmission system as claimed in claim 13 wherein the means for switching the transmission signal current to said second current loop of opposite polarity comprises means in said first loop for detecting and indicating the cessation of current flow therein, and means responsive to the indicated cessation of current flow for switching connection of the transmission line between two circuits containing oppositely directed voltage sources. 
     
     
       15. A signal transmission system as claimed in claim 14 wherein the two circuits containing oppositely directed voltage sources comprise parallel circuits each containing an individual voltage source and the switching means comprises a single switch for connecting the selected circuit. 
     
     
       16. A signal transmission system as claimed in claim 14 wherein the two circuits containing oppositely directed voltage sources comprise a single voltage source and the switching means comprises a pair of switches for connecting the single voltage source in opposite senses into the circuit.